1. Field of the Invention
The present invention relates to a method for a dynamic or real time control of a steel making process involving decarburization of molten steel under atmospheric pressure and formation of an exhaust gas comprising CO, CO.sub.2 and N.sub.2. Particularly, the present invention relates to such a method wherein the carbon content of the steel at the end point of the decarburization process may be precisely controlled to a preset value by promptly detecting the carbon content or rate of decarburization of the molten steel being processed at any desired instance and by controlling the process in accordance with the detected carbon content or rate of decarburization.
2. Brief Description of the Prior Art
Widely practiced is a steel making process under atmospheric pressure which involves decarburization of molten steel. In such a process, molten steel is decarburized in a top- or bottom-blown converter, or in an AOD (argon-oxygen decarburizing) furnace, to a preset level of carbon content to produce an intended steel.
Recent progress in the art has made it possible to produce various kinds of steel, and in consequence, it has become increasingly important to promptly detect and determine certain parameters indicative of the state of the molten steel being processed and to control the process in accordance with the determined values of the parameters so that the desired steel may be produced. Among others detection of the carbon content of the molten steel being processed is particularly important, because the primary object of the process is to decarburize the molten steel.
Determination of the carbon content in the molten steel being processed may be carried out by stopping blowing of oxygen, sampling the molten steel (in the case of a converter, after inverting the converter), and analysing the sampled steel as promptly as possible. However, such a sampling of the molten steel and analysis thereof are extremely burdensome operations. It would be advantageous from view points of both process efficiency and product quality, if it is possible to precisely determine the carbon content of the molten steel being processed without the need of sampling of the molten steel and subsequent analysis thereof.
An approach to the problem is based on measurement of the amount of carbon which has been transferred to the exhaust gas, that is the quantities of CO and CO.sub.2 in the exhaust gas, instead of measurement of the amount of carbon remaining in the molten steel. For this approach it is essentially required to precisely detect not only the quantities of CO and CO.sub.2 in the exhaust gas but also the quantity of the entire exhaust gas every moment. There has been an attempt to determine the carbon content of the molten steel being processed by monitoring the flow rate of exhaust gas by means of a differential pressure flowmeter provided in an exhaust gas duct and by monitoring the contents of CO and CO.sub.2 in a sample of the exhaust gas by means of infrared gas analyzers. However, no satisfactory results have been obtained by such an attempt. This is partly because an extremely large amount of an exhaust gas is allowed to flow through a duct of a large cross-section in a steel making process under atmospheric pressure, and the temperature of the exhaust gas extensively varies in the course of the process, making it difficult to precisely detect the quantity of the exhaust gas with an instrument such as a differential pressure flowmeter, and; partly because analyzers such as an infrared gas analyzer had a limited precision and response speed. Furthermore, the fact that different infrared gas analyzers are required for detecting different gaseous components in a sample of the exhaust gas poses difficulties in handling errors and time-lags of the respective analyzers. Accordingly, it has been very difficult to precisely determine the content of carbon in the molten steel being processed from information about the exhaust gas every moment. Values of the carbon content found by the prior art method frequently fluctuate to a great extent, and a probability with which the determined values make a good hit with acceptable precision is in the order of about 60 to 80%. Particularly, in a steel making process carried out under atmospheric pressure, atmospheric air inevitably entering the exhaust gas affects the quantity of the gas and makes it further difficult to obtain precise information on the molten steel from that on the exhaust gas.
In Japanese Patent Laid-open Specification No. 50(1975)-99592, published on Aug. 7, 1975 and assigned to the assignee of the present application, we have disclosed a method of determining the quantity of a gas formed in a gas producing chamber, such as the quantity of steam formed in a drier. The method proposed therein comprises the steps of feeding a dummy gas to the gas producing chamber, monitoring the quantity of the dummy gas fed to the gas producing chamber as well as the partial pressures of the dummy gas and the gas formed contained in an exhaust gas, and determining the quantity of the gas formed from the monitored values. The laid-open specification further teaches that the partial pressure of the gases may be advantageously measured by a mass spectrometer, and suggests that the proposed method may be applicable for determination of gases formed in a steel making furnace. However, this laid-open specification is completely silent with respect to difficulties inherently involved in mass spectrometrical analysis of a gas comprising CO, CO.sub.2 and N.sub.2. In fact, parent peaks for CO and N.sub.2 in a mass spectrum are inseparable because CO and N.sub.2 have the same mass number of 28. Furthermore, a fragment peak for CO.sub.2 appears at a mass number of 28 and perturbs the parent peak for CO which also appears at the same mass number.